1. Field of the Invention
The present invention relates to AC/DC rectifiers.
2. Description of the Related Art
In a typical "buck derived" full-bridge DC/DC converter, the DC input is chopped into a pulse-width modulated AC signal which is then voltage-converted by a transformer. The output of the transformer is then rectified by a full-wave diode bridge rectifier.
In such a design, a concern about the durability of the rectifier diodes can arise. During an "active" portion of the waveform being provided to the rectifier, two diodes conduct and two are in a blocking state. However, when the waveform is in a "dead" portion of its PWM cycle, all four diodes "freewheel" because current continues to flow to the load. That freewheeling current flows through all four diodes of the rectifier bridge. When the waveform enters an "active" portion again, two of the diodes become reverse-biased and must "commutate" to blocking mode. When a diode commutates to blocking mode, the charge within the diode must be expelled from the diode. The charge leaves in the form of a temporary flow of reverse current through the diode. This reverse current causes, in combination with the reverse voltage applied across the diode, "reverse recovery" power dissipation.
It has been observed that the recovery power dissipation can be significant enough to not only be energy-wasteful, but also to be destructive to the bridge rectifier diodes. In a particular DC/DC converter application in an electric vehicle, the reverse recovery power has been measured at a peak value of up to three kilowatts. Where the switching frequency of the converter is high, this amount of power dissipation can cause destructive heating of the diodes.
Therefore, a system which can reduce the reverse recovery power dissipation in the bridge rectifier diodes can improve their durability and thereby provide advantages over the prior art.